As is known, operational amplifiers are amplifiers based on the principle of the differential amplifier and realized as integrated, in particular monolithically integrated, semiconductor circuits. According to the reference book by U. Tietze and Ch. Schenk, "Electronic Circuits, Design and Applications", 1991, ISBN 0-387-50475-4, pages 118 to 123, either a circuit arrangement with the property of a non-inverting amplifier or a circuit arrangement with the property of an inverting amplifier can be realized with an operational amplifier, by virtue of the resistor circuitry of its two inputs and of the output.
In the case of the inverting amplifier, a first resistor having the value r.sub.1 connects the output of the operational amplifier to the inverting input of the latter. Furthermore, a second resistor having the value r.sub.2 is connected between the input of the circuit arrangement and the inverting input of the operational amplifier. The common junction point of the two resistors is therefore connected to the inverting input of the operational amplifier. Its non-inverting input is connected to the circuit zero-point.
The gain v of this inverting amplifier is as follows, the minus sign indicating the phase shift of 180.degree. between the input and output signals: EQU v.varies.-r.sub.1 /r.sub.2 ( 1)
In the case of the non-inverting amplifier, on the other hand, a first resistor having the value r.sub.1 ' is connected to the output of the operational amplifier. A second resistor having the value r.sub.2 ', which leads to the circuit zero-point, is connected in series with said first resistor. The common junction point of the two resistors is connected to the inverting input of the operational amplifier, and its input is identical to that of the non-inverting amplifier. The gain v' of this amplifier is as follows; EQU v'=1+r.sub.1 '/r.sub.2 ', (2)
in other words is positive, which indicates phase coincidence between the input and output signals.
Compared with the inverting amplifier explained above, the non-inverting amplifier has the disadvantage that its gain can only be equal to or greater than one.
If the two resistors are also incorporated in an integrated semiconductor circuit as mentioned above, then it in, moreover, difficult to obtain gain values of between 1 and 1.25, since unfavorable resistor values are necessary for this.
In the case of these small gains, moreover, a further disadvantage of the non-inverting amplifier arises, which is based on the property of the latter that the respective potential of the two inputs is equal to the input voltage of the amplifier. Consequently, each input must be able to follow the input voltage, the maximum value of which may be considerable. The input voltage range which, on the other hand, can still just be processed by the amplifier is referred to as its common mode input range.
In the case of said small gains, it is then possible to realize necessary, largo values of the common mode input range only with difficulty, if at all. Since, furthermore, today's integrated semiconductor circuits frequently comprise digital subcircuits, which process digital signals and have a digital circuit zero-point corresponding to a first reference potential, and analog subcircuits, which process analog signals and have an analog circuit zero-point corresponding to a second reference potential, in the case of the non-inverting amplifier the current in the second resistor flows to the potential of the analog circuit zero-point.
Only when the latter has a sufficiently low series resistance with respect to the main circuit zero-point of the integrated semiconductor circuit does the current also flowing therein not cause an interference voltage superposed on the desired potential of the analog circuit zero-point. However, the necessary, low series resistance can often not be realized owing to other conditions of the integrated semiconductor circuit which have to be complied with.